With the increasing availability of efficient and low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication terminal. The different modulation and transmission schemes each have advantages and disadvantages. In a wireless network, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one or more base stations communicating with terminals within the cell. When the user of a terminal moves between cells, or the radio coverage areas associated with different base station sites, an ongoing call must be switched to a different radio coverage channel or cell. The base station transmits signals to the terminals within the cell in down link and these terminals transmit signals to the base station in uplink.
Duplex in wireless networks relates to how signals, transmitted in the downlink and uplink, are multiplexed. In Frequency Division Duplex (FDD) systems, downlink and uplink signals are transmitted in different frequency bands, separated by a duplex distance. The duplex distance is often larger than the downlink and uplink signal bandwidths, resulting in a pair of spectrum allocations with a gap in between. This spectrum allocation is often referred to as paired spectrum. In Time Division Duplex (TDD) systems downlink and uplink signals are transmitted at the same frequencies but at different instants in time. E.g. Wideband Code Division Multiple Access (WCDMA) and IEEE 802.16-based systems can operate in both FDD and TDD modes.
A benefit of TDD systems is that the channel is reciprocal, i.e. the channel is the same in both uplink and downlink. This means e.g. that a terminal that can estimate the downlink channel also knows the uplink channel. This can be used for e.g. accurate power control, link adaptation, and antenna weight selection.
In FDD systems, the downlink and uplink channels are different because of the frequency separation between downlink and uplink bands, which leads to different multi path fading. A problem is how to perform uplink channel estimation for FDD systems. One way of solving the problem is to use reference signals, also called pilots. Each of the terminals within the cell sends a known reference signal with known amplitude/s and phase reference/s to the base stations. The base stations perform channel estimation by measuring each of the received reference signals. Channel estimation may e.g. be done by measuring the amplitude(s) and phase(s) of the received reference signals and comparing the results with the known transmitted signal, i.e. comparing with the known amplitude and phase reference, and in that way the base station generates estimates of the uplink channels. The base stations then either report this result back to each of the terminals within the cell, and/or uses the result themselves to schedule the users in appropriate parts of the band, select power levels, modulation and coding schemes etc., and report the decisions to the terminals. The FDD modes of WCDMA and 802.16 use the above principle which principle for example is described in US2006018368.
A disadvantage of this approach is that it involves a significant overhead. Uplink pilots have to be transmitted by and reported back to all terminals within the cell. If the uplink pilots are used for channel dependent scheduling, link adaptation, or power control in the frequency domain, pilots must basically cover the whole uplink bandwidth, which causes a large overhead and is time consuming. Once the report arrives at the terminal, the channel might have changed.